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Wave-Particle Duality

Wave-Particle Duality. What is LIGHT?. Ancient Greeks had 2 lines of thought. 1. Emission theory: light is a torrent of super small, super fast particles . 2. Aristotle added a fifth element to the 4 elements of nature (fire, air, earth, water) and called it aether .

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Wave-Particle Duality

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  1. Wave-Particle Duality

  2. What is LIGHT? • Ancient Greeks had 2 lines of thought. 1. Emission theory: light is a torrent of super small, super fast particles. 2. Aristotle added a fifth element to the 4 elements of nature (fire, air, earth, water) and called it aether. • No such thing as empty space, it’s filled with aether and vision “arises from a movement, produced by the body we perceive, in the interposed medium.” • I.e., Light was aethereal motion.

  3. Wave • 1660s  Robert Hooke (while studying color patters on transparent films) developed the basic wave theory. • Light must be very fast vibrations of the aether with very high frequency. • It’s faster than sound (fastest thing known at the time) and therefore, the only reasonable explanation is that light is a wave. • It’s super fast and therefore, must be energy transported without matter.

  4. Particle • Newton tried to stay out of it all. But it seems he leaned towards the particle view. • The straight line reflection of light seems to promote a particle.

  5. Dual Nature? • Knowing Hooke’s experiments, Newton did what Newton do. • Maybe light is a stream of particles, but these are capable of creating vibrations in the aether. • Waves of aether guide particles of light.

  6. No, it’s a wave • Early 1800s  Thomas Young and others bring back the wave theory with lots of evidence. • Young’s Double Slit Experiment

  7. Mid 1800s James Clerk Maxwell discovered that a time varying magnetic field creates an electric field (hence the term, electromagnetic wave). • Fun fact: from Kirkcudbrightshire, Scotland. • Maxwell’s Equations (4 of them) described light as a wave. And they worked. • Between Young and Maxwell, particle theory was on the out and out.

  8. Wait…it’s a particle • Late 1800s  Heinrich Hertz observed the photoelectric effect.

  9. Shining a light on certain metals will induce an electric current. • Light knocks electrons out of the metal, resulting in current. • Certain metals only react to certain colors, regardless of intensity.

  10. 1905  Einstein, building off work done earlier by his mentor (Max Planck), described light as packets of energy, called photons. • The energy of ejected e- are proportional to the frequency of the illuminating light. • Photons must be acting like a particle that is giving all of it’s energy to the e-. • Only photons of high enough frequency (above a certain threshold level) could knock an electron free.

  11. So what is light? • Neither. • Sometimes it acts like a wave. Sometimes it acts like a particle. • Depends on what you’re observing and how you’re observing it.

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